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‫Hello and welcome.

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‫In this lecture, we're going to be learning about line traces and sphere sweeps, which are going to

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‫help us be able to query the world from our code.

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‫We're also going to set up a collision channel so that we can have objects respond to the grabber or

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‫not respond to the grabber.

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‫Let's dive in and see how this works.

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‫So as far as our code is concerned, it's pretty much blind.

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‫It cannot tell what I am looking at right now and it cannot tell whether I am trying to pick up a gargoyle

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‫or whether I might be trying to pick up something else entirely else.

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‫As far as the code is concerned, it's standing in a pitch black room and it cannot see anything.

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‫So how do we solve the problem of figuring out from code what is out there?

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‫Well, essentially, we need to switch on a high beam flashlight and look in a certain direction to

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‫see what there is.

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‫Essentially flash a light into a dark scene.

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‫So imagine that hidden in our dark scene.

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‫We've got these shapes here.

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‫What we might do is take a starting point and an ending point and essentially trace a line through from

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‫the start to the end and see what that line overlaps with.

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‫This called a line trace.

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‫So when you do a line trace, you essentially can get back either a result, which is all of the objects

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‫that the line intersected with on its path to the end.

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‫Or you can just get the first object that it hit and some optimizations that mean that if it hits an

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‫object, it actually doesn't do the entire trace at all.

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‫So this is how we often do things like shooting guns in first person shooters or even like we're doing

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‫here, picking up objects.

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‫There's an alternative to the line trace, however, because the line trace can mean that we have to

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‫be extremely accurate.

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‫If we miss an object by a few millimeters like this sphere, it won't be picked up.

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‫So the alternative is a shape trace and sometimes more specifically known as a sphere trace.

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‫If that's the shape we're using or more generally it's actually known in code as a geometry trace.

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‫What we do here is we take a sphere starting at our starting point, and as we as name would suggest,

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‫we trace it or we sweep it, actually, we sweep the sphere through the world to the end point.

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‫And if it intersected with anything along the way, then that is something that the trace will hit.

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‫So for example, we will overlap here with this triangle and the sphere because we've expanded the radius

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‫of the search space.

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‫Now there's one more additional thing that we can add to our traces to make them ignore certain shapes

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‫in the scene.

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‫Because, for example, we know that we can't grab hold of this wall and maybe we want to be able to

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‫grab something through the bars, even though we might be hitting the bars as we do so well.

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‫The way to do that is using something called the Trace Channel, and this essentially creates a list

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‫of objects that can respond to the Trace and others will that might want to ignore it entirely.

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‫So let's take a look at how to set up our own trace channel so that we can pick up objects in our scene.

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‫Now, the place to go and do this is over in the settings.

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‫Project settings.

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‫Then go to engine and collision because this is all about object collision.

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‫And in here you can see we have got objects, channels and we have got trace channels.

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‫We're looking at particularly the Trace Channel because what we want to do is figure out how we should

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‫respond to a certain type of query.

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‫If we go back into our scene and click on an object and if we scroll down to an area called Collision

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‫and there's a bunch of settings here about how this object should respond to other objects in terms

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‫of colliding with them.

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‫So if we go at the moment that's got a default collision preset, if we have a look here, there's a

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‫lot of different collision presets here.

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‫But let's go to the custom one so we can see what is actually involved in collision settings.

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‫So we've got an object type, we've got the types of query it responds to, which is query.

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‫In this case, that's things like line traces and sphere trace and sweeps and so on.

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‫And then there's physics.

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‫So that's the simulation of physics and whether it bounces off other objects.

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‫But further down you can see there's the collision responses and this is split into trace responses

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‫and object responses.

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‫So what it's saying is here are some of the trace channels that are already built in.

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‫There's a visibility trace channel.

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‫This is basically all visible objects.

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‫And there's a camera trace channel, which is basically whether the camera should be allowed to penetrate

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‫through this object or not.

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‫But it's isn't too important.

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‫These are the types of channels that we have and we can respond to them differently.

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‫You can see here that we can have a block option.

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‫There's an overlap, which is an intermediate option and a completely ignore option.

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‫So this is what we're interested in.

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‫We're interested in adding in a new trace response channel here.

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‫So let's go over to our project settings, go to our trace channel.

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‫And Click Add New Trace Channel.

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‫Then we are going to go in here and call it grabber and we get to choose how by default objects are

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‫going to respond to this channel.

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‫I want it to ignore this channel by default so that we have to specifically say that an object responds

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‫to this trace channel.

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‫I've got to go and click Accept, go over back into our dungeon and you can see that we're not actually

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‫showing the new trace response just yet.

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‫So what we're going to do is close down the editor, save the scene and relaunch from the new project

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‫file.

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‫So once that's launched up, let's select our gargoyle again and go down to our collision settings.

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‫And sure enough, here we've got our new trace response channel grabber, which for the gargoyle we

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‫want it to block basically, meaning that it will block the line of sight and so it will show up in

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‫either our sphere sweep or our line trace.

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‫Great stuff.

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‫Hopefully now you've got a better understanding of line traces and sweeps.

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‫I'll see you in the next lecture.